2 coats of future should help, & leave pinned flat for as long as possible.
If they have yellowed, then pin up in a sunny window away from the glass surface to bleach back to clear.
You will have to cut the decal from the sheet as close to the decal as possible, as the future will bond to the un-decalled part of the sheet.
Consider that, if the decals are 16 years old the adhesive may also degrade, so make sure the surface to be decaled is clean, & glossy, & don't over soak the decal to get it off the sheet., Otherwise it's aftermarket decals.